Add functions to test properties of default Lua metamethods

Lua has a number of default metamethods (Lua 5.3 Manual -> 2.4 – Metatables and Metamethods) that defines an object behaviour for example for default Lua operators. Most of these metamethods can be covered by general properties. For example if a and b have metamethod __eq then (a == b) == (b == a) and (a == b) ~= (a ~= b), for metamethod __lt - (a > b) == (b < a) and so on. I believe it would simplify testing a bit if lua-quickcheck will discover available metamethods and them with such properties.

What do you think, @luc-tielen? I'm ready to spend some time implementing this.

General methods

__index - none?
__newindex - none?
__mode - none?
__call - none?
__metatable - none?
__tostring
```
type(tostring(a)) == 'string'
```
__len
```
type(#a) == 'number' 
```
__pairs - none?
__ipairs - none?
__gc - none?
__name
```
type(a.__name() == 'string'
```
__close

Mathematic Operators

__unm
```
-(-a) == a
```
__add
```
a + b == b + a
a + 0 == a
```
__sub - none?
__mul
```
a * 2 == a + a
a * 1 == a * 1 * 1
```
__div - none?
__idiv - none?
__mod
```
a % b == a - math.floor(a / b) * b
```
__pow
__concat - none?

Bitwise Operators

__band
```
(a & (a - 1)) == 0, if a is x^2
```
__bor
```
a | 0 == a
```
__bxor
```
a && b == b && a
```
__bnot
```
~(~a) == a
```
__shl
```
a << 0 == a
```
__shr
```
a >> 0 == a
```

Equivalence Comparison Operators

__eq

(a == b) == (b == a)
(a == b) ~= (a ~= b)

__lt
```
(a > b) == (b < a)
```
__le
```
(a >= b) == (b <= a)
```

luc-tielen / lua-quickcheck